Can I cruise if I'm pregnant?

No, not really. You can get pregnant if you want but you can't cruise. Well not entirely........
You can sometimes subject to certain terms and conditions slightly different for each cruise line.

On the whole most cruise lines will say they are happy for you to cruise if you're pregnant as long as you finish the cruise before your 21st week. Past this most cruise lines will just say no to you. Some cruise lines may let you sail up to about 28 weeks but these are rare so if you're unsure it's always worth checking the terms and conditions of that particular cruise line, (see here), or just call up your favourite travel agent, (me hopefully), and have them do it.
This may seem unfair but there are very good reasons behind it that are designed with your health and safety in mind.
All cruise ships have emergency medical facilities onboard but they are obviously no where near as good as their land based equivalents. If anything did happen whilst you were onboard and you went into labour the ship just wouldn't have the facilities to look after a premature baby so the cruise lines have made the decision to just not let women that are to heavily pregnant travel and I can't say I blame them.
Another thing worth bearing in mind is that if you book a cruise for say next year and then find you have fallen pregnant your money is still non-refundable. The cruise line might allow you to transfer your cruise to a later one, subject to their terms and conditions, and they're normally quite flexible on this but they wont let you cancel free of charge.

Happy cruising.